In general, it should never be necessary, given that cats are cleanliness fanatics. But accidents happen: they get splashed with mud or motor oil, fall into a puddle—in short, they need cleaning that their tongues just can’t handle.
Needless to say, bathing a cat (especially one that is not declawed) is a daunting task, but some owners do it often, and apparently cats can learn to tolerate being bathed. Suggestion: arm yourself with gloves and an apron, and pray for patience. (Note: A few breeds, including the Turkish Van, actually seem to like bathing.)